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Introduction

One remarkable, and to some extent unexpected, outcome of the Voyager mission is that after the Jovian and Saturnian encounters, the science that Voyager was accomplishing became as much the science of the interstellar medium as of the solar wind. This development was further confirmed by the Ulysses mission, which began to explore the direct coupling of the interstellar medium to the solar wind through the intermediaries of pickup ions and interstellar gas. It is not widely recognized that by mass the interplanetary medium beyond 10 AU is in fact dominated by neutral atoms of interstellar origin rather than by solar wind protons (Gruntman, 1993). Being uncharged, however, the interstellar neutrals are not coupled directly to the solar wind. Instead, the coupling proceeds indirectly, through the ionization of the neutrals by various mechanisms (photoionization, charge-exchange, electron-impact ionization; see Zank, 1999a). Consequently, the physics of the outer heliosphere beyond 10 AU is very different from that in the inner heliosphere, which is determined by material of solar origin. Thus, exploration of the outer heliosphere offers the opportunity to learn about both the interplanetary and the interstellar medium, and the manner in which they interact.

Although considerable effort has been expended observationally and theoretically, understanding of the physics of the outer heliosphere remains limited (see Zank, 1999a, for a comprehensive review). The detailed interaction between the local interstellar medium (LISM)1 and the solar wind is still not well understood, for reasons that range from incompletely formulated physical models to currently poor knowledge of many of the pertinent physical parameters of the LISM. For example, we do not know whether the LISM flow is super- or subfast magnetosonic or even whether it is supersonic or subsonic when galactic cosmic rays are included, which implies that we do not know the basic morphology of the heliosphere—that is, whether it is a two-shock model (bow shock plus termination shock; Baranov et al., 1971) or a one-shock model (termination shock only; Parker, 1963). See Figure 2.1 in Chapter 2.

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The LISM is that region of space in the local galactic arm where the Sun is located (Thomas, 1978), the local interstellar cloud is the cloud within it in which the Sun resides, and the heliosphere is the region in space filled with solar wind material (both supersonic and subsonic flow).



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Exploration of the Outer Heliosphere and the Local Interstellar Medium: A Workshop Report 1 Introduction One remarkable, and to some extent unexpected, outcome of the Voyager mission is that after the Jovian and Saturnian encounters, the science that Voyager was accomplishing became as much the science of the interstellar medium as of the solar wind. This development was further confirmed by the Ulysses mission, which began to explore the direct coupling of the interstellar medium to the solar wind through the intermediaries of pickup ions and interstellar gas. It is not widely recognized that by mass the interplanetary medium beyond 10 AU is in fact dominated by neutral atoms of interstellar origin rather than by solar wind protons (Gruntman, 1993). Being uncharged, however, the interstellar neutrals are not coupled directly to the solar wind. Instead, the coupling proceeds indirectly, through the ionization of the neutrals by various mechanisms (photoionization, charge-exchange, electron-impact ionization; see Zank, 1999a). Consequently, the physics of the outer heliosphere beyond 10 AU is very different from that in the inner heliosphere, which is determined by material of solar origin. Thus, exploration of the outer heliosphere offers the opportunity to learn about both the interplanetary and the interstellar medium, and the manner in which they interact. Although considerable effort has been expended observationally and theoretically, understanding of the physics of the outer heliosphere remains limited (see Zank, 1999a, for a comprehensive review). The detailed interaction between the local interstellar medium (LISM)1 and the solar wind is still not well understood, for reasons that range from incompletely formulated physical models to currently poor knowledge of many of the pertinent physical parameters of the LISM. For example, we do not know whether the LISM flow is super- or subfast magnetosonic or even whether it is supersonic or subsonic when galactic cosmic rays are included, which implies that we do not know the basic morphology of the heliosphere—that is, whether it is a two-shock model (bow shock plus termination shock; Baranov et al., 1971) or a one-shock model (termination shock only; Parker, 1963). See Figure 2.1 in Chapter 2. 1   The LISM is that region of space in the local galactic arm where the Sun is located (Thomas, 1978), the local interstellar cloud is the cloud within it in which the Sun resides, and the heliosphere is the region in space filled with solar wind material (both supersonic and subsonic flow).

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Exploration of the Outer Heliosphere and the Local Interstellar Medium: A Workshop Report The tangible manifestations of the interaction are the completely unexplored boundary regions between the solar wind and the LISM. The boundary regions are separated by the largest shocks in the solar system, of which at least one may be the site where cosmic rays are accelerated, thereby providing a link to the supernova shocks thought to accelerate galactic cosmic rays (see, for example, Blandford and Eichler, 1987). A heliopause or tangential discontinuity is another expected boundary, where the solar wind borders the interstellar plasma and the particle number density changes. An enormous inner heliosheath perhaps 50 AU wide with plasma temperatures of 106 K, containing a wall of amplified magnetic field and a possibly unstable current sheet (Washimi and Tanaka, 1996; Linde et al., 1998; Pogorelov et al., 2004), with perhaps very high levels of magnetic reconnection and associated particle acceleration, is bounded by the termination shock and heliopause. The termination shock itself may act as a gigantic emitter of shocks and transients, and an outer heliosheath will most likely be present, bounded possibly by a bow shock that may be modified by the interaction of interstellar neutrals. On the side toward the Sun’s motion through the LISM, the outer heliosheath will contain a region of heated, compressed, and slowed neutral hydrogen—the “hydrogen wall” (Baranov and Malama, 1993; Pauls et al., 1995). Multiple species of atoms will be present, possessing very diverse thermal properties, reflecting the complications associated with charge exchange in a highly non-equilibrated boundary region. Since neutral-atom/plasma charge-exchange mean free paths are very long compared with heliospheric length scales, simple equilibrium models of neutral hydrogen and plasma are an inadequate description for the boundary regions of our heliosphere. Exploring this vast and complex region are two venerable spacecraft launched more than 26 years ago that constitute the Voyager Interstellar Mission.2 Because the boundary regions (the boundaries themselves, the regions bounded by discontinuities, the hydrogen wall, the physics of the partially ionized plasma, and so forth) of our heliosphere are completely unexplored, the Voyager Interstellar Mission promises a continuing rich harvest of scientific results. Given that some 26 years passed before the Voyager 1 spacecraft reached a distance of 90 AU from the Sun, it is highly unlikely that the solar wind-LISM interaction of our heliosphere can be explored in situ by spacecraft in the next 20 years—effectively making the Voyager Interstellar Mission irreplaceable in the view of workshop participants. Sending a well-equipped spacecraft to the boundaries of our heliosphere to begin the exploration of our galactic neighborhood will be one of the great scientific enterprises of the new century—one that will capture the imagination of people everywhere. Interstellar space is a largely unknown frontier that, along with the Sun as the source of the solar wind, determines the size, shape, and variability of the heliosphere, the first and outermost shield against the influence of high-energy cosmic rays. The interstellar medium is the cradle of the stars and planets, and its physical state and composition hold clues to understanding the evolution of matter in our galaxy and the universe. With plentiful bodies of all sizes and dust in the Edgewood-Kuiper Belt and in the Oort Cloud, the outer heliosphere is a repository of frozen and pristine material from the formation of the solar system. After the contents of our solar system, which is 4.5 billion years old, the LISM provides a second, more recent, sample of matter in our galaxy and in fact the only sample of the interstellar medium that can be studied close-up and in situ. Last but not least, the heliosphere is the only example of an astrosphere that is accessible to detailed investigation (Linsky and Wood, 1996; Gayley et al., 1997). These perspectives provide a natural bridge and synergism between in situ space physics, the astronomical search for the origins of life, and astrophysics. In general terms, the four principal science objectives of a mission to the border of our galaxy are as follows:3 2   Voyager 2 was launched on August 20, 1977, and Voyager 1 on September 5, 1977, from Cape Canaveral, Florida, aboard Titan-Centaur rockets. See the Voyager Web site at http://voyager.jpl.nasa.gov/. 3   See Mewaldt and Liewer (2001) and http://interstellar.jpl.nasa.gov/interstellar/probe/index.html.

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Exploration of the Outer Heliosphere and the Local Interstellar Medium: A Workshop Report To explore the nature of the interstellar medium and its implications for the origin and evolution of matter in our galaxy and the universe; To explore the influence of the interstellar medium on the solar system, its dynamics and its evolution; To explore the impact of the solar system on the interstellar medium as an example of the interaction of stellar winds with their environments; and To explore the outer solar system for clues to its origin, and the nature of other planetary systems. A spacecraft for such explorations, such as Interstellar Probe,4 will require an advanced propulsion system as well as sophisticated communications and instrumentation, if we are to take our first halting steps to the edge of and beyond our solar system birthplace.5 These objectives are further elaborated in the chapters that follow. 4   See http://interstellar.jpl.nasa.gov/interstellar/probe/index.html. 5   “The Earth is a cradle of the mind, but we cannot live forever in a cradle.” Attributed to Konstantin E. Tsiolkovsky, the Father of Russian Astronautics, 1896. See http://vesuvius.jsc.nasa.gov/er/seh/quotes.html.